Physical capacity and muscular function are reduced with aging. Age-associated reductions in muscle blood flow and its intravascular distribution within and between skeletal muscles is thought to be linked mechanistically to muscle dysfunction in older individuals. To date, however, key structural and functional relationships within young and aged skeletal muscle remain obscure. Furthermore, as there is an age- associated increase in the percentage of adipose tissue, altered vasoreactivity in this tissue bed may affect whole body hemodynamics and skeletal muscle blood flow in the aged population. I plan to test the broad hypothesis that aging induces microcirculatory dysfunction (both structural and functional) that impairs oxygen transport, and exercise training can attenuate this microcircular dysfunction. This work will utilize the rat model of aging that is a widely accepted analog of the human condition that has the major advantage of permitting very invasive studies. The overall goal of this application is to provide me with new and complementary skills to become an independent researcher, and provide a foundation to launch my scientific career. In this regard each specific aim will allow me to master a novel technique to address the following in young, middle-aged, and old rats: 1) Elucidate the structural (e.g., vascular geometry), functional (e.g., endothelium-dependent vasodilation with tetrahydrobiopterin or L-arginine incubation), and molecular characteristics (e.g., eNOS mRNA and protein expressions) of skeletal muscle and adipose tissue resistance vessels via the isolated microvessel technique, 2) Examine alterations in skeletal muscle vasculature resulting from prolonged exercise training, 3) Transiently alter eNOS, VEGF or GTP cyclohydrolase mRNA and protein expression via adenovirus gene therapy, and 4) Determine whether detremints in adipose tissue vasoreactivity with age may contrribute to reduce skeletal muscle blood flow during exercise. Development of effective strategies to attenuate the deleterious effects of aging on physical capacity is contingent upon resolution of the mechanistic bases for muscle dysfunction in this population. In this regard, these specific aims seek to address several important potential mechanisms for the physical limitations in senescent individuals, and provide the basis for future research and funding to initiate an independent career.